1. Field of the Invention
This invention relates to a sliding feed mechanism having a worm gear, and in particular relates to an improvement of a sliding feed mechanism for an optical pick-up used in an optical disc drive such as CD-ROM drives and CD-R drives and the like.
2. Description of the Prior Art
Sliding feed mechanisms having a worm gear comprised of a worm and a worm wheel are well known as one of the mechanisms for converting a rotational motion into a linear motion. Since such sliding feed mechanisms can obtain a large speed reduction ratio and noise generated therefrom is relatively small, such mechanisms are widely used in various machines.
For example, in disc drives such as CD-ROM drives, a sliding feed mechanism is used for moving an optical pick-up for reading out information recorded on the CD-ROM in a radial direction of the CD-ROM.
In general, the sliding feed mechanism used in the disc drive is roughly constructed from a DC motor capable of forward and reverse rotations, a worm which is mounted on a rotational shaft of the DC motor, a worm wheel which has a rotational axis orthogonal to the rotational shaft of the motor and meshes with the worm of the motor, a small-diameter pinion gear which rotates together with the worm wheel, a rack gear which is in engagement with the pinion gear, and a slider or a carriage (driven means) which is mounted to the rack gear and which is provided so as to be slidable in the axial direction of the rotational shaft of the motor along a guide rod in accordance with the movement of the rack gear. An optical pick-up is provided on the slider. With this construction, the optical pick-up moves in the radial directions of the optical disc by rotating the motor in either forward or reverse direction.
In such a sliding feed mechanism for moving the optical pick-up, a small DC motor with a low torque is used in order to reduce an amount of consumed electrical power and a space for installing the motor. Further, due to the necessity of fine control for the optical pick-up, a worm having a short pitch and a small lead angle is used in the worm gear of the sliding feed mechanism.
Meanwhile, in recent years, disc drives are improved so as to be able to rotate a disc at high speed such as 8 times speed, 12 times speed or more. In such disc drives in which the disc is rotated at such a high speed, it becomes necessary to increase accessing speed of the optical pick-up to a designated track, for example.
In order to increase the accessing speed, one approach is to increase rotational speed of a motor used in a sliding feed mechanism for an optical pick-up. However, this approach involves a problem in that an amount of current used for driving the motor becomes larger, which results in a large amount of consumed electrical power. Another approach is to increase an amount of movement of the optical pick-up per one rotation of the motor while reducing the number of rotation of the motor. However, this in turn creates a problem in that it becomes difficult to carry out fine control of the optical pick-up.
Further, in the prior art sliding feed mechanism for an optical pick-up, there is a case that the worm is locked up on the worm wheel when the optical pick-up is moved to the innermost position or the outermost position due to the size of the lead angle of the worm or the structure of the rack gear. Specifically, in these sliding feed mechanisms, when the optical pick-up is moved to the innermost position or the outermost position, the pick-up is restricted to move further, and in this state the worm wheel is normally no longer rotated further. However, at this state, if the worm is forcedly rotated by the motor, there is a case that slipping or the like would occur between the teeth of the worm and the teeth of the worm wheel engaged therewith to cause meshing interference between the worm and the worm wheel, which results in a locking state therebetween. In particular, in the sliding feed mechanisms for an optical pick-up, since a worm having a short pitch and a small lead angle is used as described above, such a locking state is likely to occur.
If such a locking state would occur, there is a case that the worm can not be returned to the normal engaging condition from the locking state since the motor can not be rotated in the reverse direction because of the small-size low torque motor being used in the sliding feed mechanism as described above.
In order to prevent such a locking state from occurring, it is the easiest way to enlarge the lead angles of the worm and the worm wheel, and this can be achieved by either increasing the number of threads of the worm or reducing the pitch circle thereof. However, this in turn creates a problem in that an amount of the movement of the optical pick-up per one rotation of the rotational shaft of the motor becomes too large, which makes it difficult to perform fine control for the optical pick-up.